SimpleTestCase does not block database connections in threads

[Daniel Hahler]

Due to ConnectionHandler's connections being thread-local [1] new connections will be used in new threads, which then do not have been patched for disallowed methods [2].

Given test_simpletestcase.py:

import threading

from django.db import connection
from django.test import SimpleTestCase


class MySimpleTestCase(SimpleTestCase):
    def test_this(self):
        try:
            with connection.cursor() as cursor:
                cursor.execute("SELECT 1")
            raise Exception("should have failed")
        except AssertionError:
            pass

        res = []

        def thread_func():
            res.append(1)
            try:
                with connection.cursor() as cursor:
                    cursor.execute("SELECT 1")
                raise Exception("should have failed")
            except AssertionError:
                pass
            res.append(2)

        t = threading.Thread(target=thread_func)
        t.start()
        t.join()
        assert res == [1, 2], res

./manage.py test test_simpletestcase.py fails like this:

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python3.9/threading.py", line 973, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.9/threading.py", line 910, in run
    self._target(*self._args, **self._kwargs)
  File "…/test_simpletestcase.py", line 23, in thread_func
    raise Exception("should have failed")
Exception: should have failed
F
======================================================================
FAIL: test_this (test_simpletestcase.MySimpleTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "…/test_simpletestcase.py", line 31, in test_this
    assert res == [1, 2], res
AssertionError: [1]

----------------------------------------------------------------------
Ran 1 test in 0.006s

FAILED (failures=1)

(Note that there is some handling of connection.settings_dict for workers of the test runner, which is only slightly related: ​https://github.com/django/django/blob/dfa1145a22042dcf9e504a5a7edd5557e3e0d07c/django/test/runner.py#L327-L335)

A possible solution might be to use the existing ​connection_created signal to raise an exception when a connections was created (although that would happen only after the fact - a new pre-connect signal could be used/added for this).

Given that the test DB names are not prefixed with test_ with SimpleTestCase you might accidentally change the production DB from within your tests when something like a ThreadPoolExecutor is being used when mixing sync with async etc.

1: ​https://github.com/django/django/blob/dfa1145a22042dcf9e504a5a7edd5557e3e0d07c/django/utils/connection.py#L41
2: ​https://github.com/django/django/blob/dfa1145a22042dcf9e504a5a7edd5557e3e0d07c/django/test/testcases.py#L183